In the quantum cryptographic communication system, loading each photon with information enables the detection of an unauthorized listener by the quantum mechanical principle. However, if the same information is loaded on two or more photons, the unauthorized listener may utilize a part of these photons and the presence of the unauthorized listener may not be able to be detected. In this way, ideally, a pulse that contains only one photon at maximum must be used. For this kind of pulse, it is popularly practiced to attenuate the light beam from the laser beam source by an attenuator in such a manner that the mean number μof photons per pulse becomes about 0.1. By doing this, the probability to contain two or more photons in a pulse can be reduced. However, the probability to contain one photon in the pulse is also reduced to about 0.1. That is, in the case of μ=0.1, transmission is actually carried out only about once per 10 times.
Referring now to FIG. 9, description will be made on the case stated in the “Key Distribution system and method using Quantum Cryptography” of Japanese Unexamined Patent Publication No. 505019/1996 as one example of conventional techniques for improving this kind of process. In FIG. 9, numeral 9 denotes a laser that generates pumping light for pumping the nonlinear optical crystal 11. In the nonlinear optical crystal 11, a parametric fluorescence pair that causes one photon of the pumping light to stochastically generate two photons is generated. One photon of these (in this case, called the “idler photon”) is detected by an optical detector and a gate controller 49, and when detected, the gate device 4 is opened to enable the other photon (called the “signal photon”) to pass.
However, in the conventional technique, there are following problems.
First of all, the conventional method has a drawback in that if two photon pairs exist within the response time of the detector, two signal photons are emitted by a gate operation and two photons exist in a pulse.
In the conventional method, it was unable to control the timing of photon generation within the pulse.
When the detector for detecting the arrival of the photon generates a so-called “dark count pulse,” that is, when the detector outputs pulses due to noises, etc. even when it does not detect the photon, it outputs non-existent light pulse in which no emission photon exists and provided poor efficiency.
The present invention has been made to solve these problems, and it is an object of the present invention to generate only one photon in one pulse.
It is another object of the present invention to reduce generation of non-existent beam pulse due to dark count pulse of the detector.
It is yet another object of the present invention to generate the photon at a specific timing.